1. Field of the Invention
The present invention relates to a method of manufacturing a vehicle seat such as an automotive seat, and in particular to an ultrasonic welding method for the vehicle seat, in which a covering member is welded to a foam cushion member by means of ultrasound.
2. Description of the Prior Art
There has been a difficulty in welding a covering member to a foam cushion member of a certain shape foamed in a mold, by use of a high frequency wave welding method. In a recent prior art, there has been proposed an ultrasonic welding method to tide over such difficulty, which is disclosed in the Japanese Laid-Open Patent Publication No. 59-230590. This first invention enables welding to a cushion member such a covering member of material and shape that have been difficult to do so by the high frequency wave method, and as a further advantage, it allows extending its welding to other covering members, including the one that is apt to be damaged by a spark occuring in the high frequency wave welding process.
However, in such ultrasonic welding method, a problem has existed in that, when an ultrasonic vibration horn having a great welding area is used, the covering member can not be welded uniformly to the cushion member at their mutual welding regions.
Our research for the cause of this problem reveals that it is attributed to an uneven state between the welding portion of the ultrasonic vibration horn and the thickness of the cushion member. In other words, in the case where the welding portion of the ultrasonic vibration horn is formed greater in length (for example, 25 cm) and unevenly formed is the thickness of the cushion member beneath such welding portion of the horn, then there is a varied pressure distribution of the horn against the cushion member due to the uneven thickness of the cushion member, with the result that the ultrasonic welding through the horn is not effected uniformly to the cushion member.